Implants have found uses in modern medical technology in manifold embodiments. They are used, for example, for supporting vessels, hollow organs, and duct systems (endovascular implants), for attaching and temporarily fixing tissue implants and tissue transplants, and for orthopedic purposes, for example, as nails, plates, or screws. Frequently, only a temporary support and/or retention function is necessary or desired until completion of the healing process or stabilization of the tissue. To avoid complications which result from the implants remaining permanently in the body, the implants must either be operatively removed or the implants must contain a material which is gradually degraded in the body, i.e., the material is biocorrodible. The number of biocorrodible materials based on polymers or alloys is growing continuously. Thus, inter alia, biocorrodible metal alloys of the elements is magnesium, iron, and tungsten are known.
European Patent Application No. 1 270 023 describes a magnesium alloy which is suitable for endovascular and orthopedic implants. The alloy may contain up to 5 weight-percent rare earth elements. The biocorrodible metal alloys and polymers for medical implants known in the art have the disadvantage, however, that the biocorrodible metal alloys and polymers are not visible or are not visible to a satisfactory extent in the current x-ray methods. However, x-ray diagnosis is an important instrument precisely for postoperative monitoring of the healing progress or for checking minimally-invasive interventions. Thus, for example, stents have been placed in the coronary artery during acute myocardial infarction treatment for some years. Currently, a catheter which carries the stent in an unexpanded state is positioned in the area of the lesion of the coronary vascular wall. Subsequently, the stent either expands by self-expanding forces or by inflation of a balloon to prevent obstruction of the vascular wall in the expanded state. The procedure of positioning and expanding the stent must be continuously monitored by the cardiologist during the procedure.
X-rays in the energy range from 60 to 120 keV are typically employed in the medical field for use on the heart, typically, but not exclusively, in the range from 80 to 100 keV. Because the x-ray absorption coefficient is strongly dependent on the energy, the operating range is to be considered when selecting suitable marker materials. The absorption (intensity attenuation) of the x-rays may be described in simplified form using an exponential attenuation law.
      I          I      0        =      exp    ⁡          [                        -                      (                          μ              ρ                        )                          ⁢        x            ]      
In the equation above, I is the measured intensity after the sample passage, I0 is the intensity of the radiation before the sample passage, μ/ρ is the mass absorption coefficient, to and x is the mass thickness of the sample. x may be calculated as the thickness t times the density of the material ρ,x=ρ*t. For alloys, the mass absorption coefficient is calculated by adding the components.
In the event of low absorption of the selected material in a given energy range of the x-ray absorption, improvement of the x-ray visibility may thus be achieved by increasing the is material thickness; however, this measure rapidly reaches its limits, in particular, when marking filigree structures, as exists in stents.
Therefore, equipping implants with a marker in the form of a coating, a strip, an inlay, or a different type of design to improve the x-ray visibility is known. For example, metal strips made of gold or other noble metals are attached in specific areas of a stent.
German Patent Application No. 103 61 942 A1 describes a radiopaque marker for medical implants, which contains 10 to 90 weight-percent of a biocorrodible base component, in particular, from the group of elements consisting of magnesium, iron, and zinc. Furthermore, the marker contains 10 to 90 weight-percent of one or more radiopaque elements from the group consisting of I, Au, Ta, Y, Nb, Mo, Ru, Rh, Ba, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf, Ta, W, Re, Os, and Bi as a marker component. The markers described are suitable in principle for use in biocorrodible implants, in particular, those made of biocorrodible magnesium alloys.
In implants made of biocorrodible metallic materials based on magnesium, iron, or tungsten, there are usually further requirements for the marker material:                the marker is not to be separated early from the main body of the implant by the corrosive processes, to avoid fragmentation and thus the danger of embolization;        the marker is not to degrade more rapidly than the main body, in order to still remain visible in later examination; however, at least partial in vivo degradation is to be provided;        the marker is to have sufficient x-ray density even at low material thicknesses; and        the marker material is to have no or only slight influence on the degradation of the main body.        
However, when markers made of metallic materials are used on biocorrodible metallic main bodies, the special problem arises that, because of electrochemical interactions between the two materials, the degradation of the main body changes in a contact area between marker and main body, and is typically accelerated. Furthermore, processing of the marker material is made more difficult because of the melting point of the base material, which is frequently low; processing methods such as soldering or laser welding, or also the immersion of the implant in a melt made of the marker material, are typically not possible.